1. Field of Invention
This invention relates to a rigid drive axle suspension system for motor vehicles.
2. Description of Prior Art
Typically, as part of an automobiles suspension, the driveshaft between the engine/transmission and the rigid drive axle gear housing is enclosed in a “torque tube.” The torque tube is rigidly attached to a drive axle/gear housing assembly at one end and the other end has a “ball” that is enclosed by “ball drive housing” that is attached to the output end of the transmission. Inside the “ball” is the universal joint that rotates the driveshaft inside the “torque tube.” At the axle gear housing the driveshaft rotates the pinion gear through a straight splined coupling.
This design has severe limitations in that the complete axle housing has to pivot from the “ball” at the transmission in all planes. This makes it difficult to control the bump and roll steer of the axle as desired.
Presently, many race cars use a modified form that incorporates a sliding or “slipper ball” end of the torque tube that allows the torque tube's length to change. This allows some design freedom, but still has limitations, in that there is no allowance for lateral shifting of the axle perpendicular to the torque tube. To overcome these shortcomings many other cars now use a torque arm with an open driveshaft. The torque arm is rigidly attached to the axle housing and extends toward the transmission. The end of the torque arm attached to the chassis near the transmission may be mounted in rubber. Sometimes it is attached to the chassis through a pivotally connected short vertical link.
Other designs typically have the torque arm attached to the chassis through a shock absorber with a parallel spring. This allows the end of the torque arm to move in all planes with the vertical plane controlled by the spring and shock absorber.
A torque arm has limitations on the length of the side view swing arm (SVSA) because of the placement of the engine/transmission in the chassis of the vehicle. It may also intrude into the body of the vehicle.
All of these designs are limited, because the length of the torque tube or torque arm is restricted by the distance between the rigid drive axle and the transmission. Because of this limitation, the torque reaction is fed into the chassis at less than one half of the wheelbase of the vehicle. This causes the torque reaction to lift the chassis both at the front and at the rear. The lifting of the chassis at the end where the drive axle is located decreases the sprung weight and increases the undamped weight on the drive axle, thereby reducing the control of the axle by the shock absorbers, when the wheels and axle move vertically over irregular terrain.
There is also a considerable change of the pinion angle in the negative direction when the axle is displaced vertically with the short torque tube or torque arm. This change in the negative direction of pinion angle causes forward rotational scrub of the tire contact patch on the ground, and when accelerating, a loss of traction. Pinion angle, usually measured in degrees is the convergent angle formed by the planes of the horizon and the longitudinal axis of the pinion gear intersecting. Zero pinion angle is when the planes are parallel. Negative pinion angle is when the planes are convergent toward the center of the vehicle and positive pinion angle is when they are convergent away from the center.
Various vehicle suspension systems have been designed to allow a vehicle to maintain traction of the drive axle tires when accelerating over irregular terrain. U.S. Pat. No. 2,300,844 (Olley) discloses a four link suspension having upper links that are shorter than the lower links. In this invention the instant center produced by the projection lines of the upper and lower links intersecting will move closer to the drive axle when the axle goes into a bump condition. The problem with this design is the side view swing arm (SVSA) is relatively short and becomes shorter with the movement of the instant center toward the drive axle which causes an increase in the pinion angle change in the negative direction and the conversion of sprung weight on the drive axle to direct vertical force or undamped weight on the tires or road wheels thus decreasing the ability of the tires to maintain traction over irregular surfaces while accelerating.
Another vehicle suspension systems is disclosed in U.S. Pat. No. 3,575,253 (Brumm) in which a rigid driven rear axle is connected adjacent each end of the vehicle body by single longitudinal link and by a spring strut consisting of a helical spring and a telescopic shock absorber. The projected line of the link intersects with the projected line of the vehicles chassis and the resulting instant center lies at a point beyond the axle opposite the drive axle. This awkward configuration results in the instant center shifting from one end of the vehicle to the opposite end when the drive axle moves into a bump condition. The problem with this design is that the when the drive axle moves into a bump condition the torque reaction of the axle when accelerating will pull the vehicle's chassis down at the drive axle decreasing available chassis clearance to the ground, drive axle clearance to the chassis and the springs and shock absorbers available stroke. Another problem with this design is that it calls for the spring strut or shock absorber bearing the axle's reaction forces in a bending mode.
Thus it is readily apparent that there is a longfelt need for a vehicle suspension system that positions the instant center closer to the opposite axle that will remain at a relatively static position or move away from the rigid drive axle toward or beyond the opposite axle during a bump condition. A significant deficiency with the previous developed solutions, as well as many other similar devices, is that they provide a vehicle suspension system that limits the ability of the vehicle's drive axle tires to maintain traction when accelerating over irregular terrain and/or puts the drive axle reaction forces on the spring strut or shock absorber rather than the axle attachment structures. The present invention satisfies the above-mentioned needs, as well as others, and overcomes the deficiencies in devices heretofore developed.